Critical End Point Behaviour Near and Away from a Tricritical Point

Measurements are reported of the intensity and the angular dissymmetry of light scattered from 'quasi-binary' and binary mixtures of ethane and heavier hydrocarbons, that have varying widths of the three-phase region $(\Delta T)$. The critical exponents $\gamma$ and $\nu$ are estimated by approaching the Lower Critical End Points (LCEP) from the two-phase region. We investigated samples of different $\Delta T$'s (from 38 to 323 mK). It is found that (a) the CEP is similar to an ordinary consolute point in terms of the values of the critical exponents and (b) there is no evidence for a critical-tricritical crossover, if one approaches the loci of LCEP's from the two-phase region to the three-phase region, even extremely close to a tricritical point $(\Delta T \rightarrow 0)$. General thermodynamic arguments are advanced to explain the similarity between a CEP and a consolute point